To maximize the transmission capacity of an optical fiber transmission system, a single optical fiber may be used to carry multiple optical signals in what is called a wavelength division multiplexed system (hereinafter a WDM system). Modern WDM systems have a high traffic capacity, for example, a capacity to carry 128 or more channels at 10 gigabits per second (hereinafter Gb/s) or more.
The optical fiber transmission system may include a relatively long trunk segment that may be terminated at a transmitting and/or receiving trunk terminal. The optical fiber transmission system may further include one or more branching units situated along its trunk path. Each branching unit (BU) may be connected to a branch segment that terminates in a transmitting and/or receiving branch terminal. Known BUs may include one or more integrated optical add/drop multiplexers (OADM). Channels could be added to and/or dropped from the trunk segment of the optical transmission system via the OADM BUs.
In such OADM systems, connections between terminals in the network may be achieved by transmitting information signals on certain channels that are dropped to a receiving branch terminal through an OADM BU, and the receiving branch terminal may add new information signals on the same channel locations where old information was previously dropped, then re-transmit the optical signals carrying new information to another receiving terminal. Channel locations that are re-used by a terminal for transmitting information between multiple terminals are referred to herein as “re-used channels.” Re-used channels allow communication of different information between different terminals utilizing the same portion of the system bandwidth. Consequently, the total network capacity can be increased.
Communication between terminals may also be achieved using broadband OADM wherein certain channels are dedicated for communication between the terminals and are not re-used for communicating between other terminals. Such channels are referred to herein as “dedicated channels.” An OADM WDM system including both re-used and dedicated channels for establishing communication between terminals in the system may be considered a hybrid OADM system since it relies on both OADM channel types.
In general the channels within the system bandwidth should fit within a bandwidth that may be reliably amplified by the amplifiers used in the system. It is desirable to implement re-used channels to increase system capacity. However, when re-used channels are provided in a system, guard bands wherein no information signals are present must be provided in the system bandwidth to allow for filtering of the channels by re-used add/drop filters.